NASA Cassini Spacecraft Captures Ocean-Like Spray At Saturn Moon

WASHINGTON -- NASA's Cassini spacecraft has discovered the best
evidence yet for a large-scale saltwater reservoir beneath the icy
crust of Saturn's moon Enceladus. The data came from the spacecraft's
direct analysis of salt-rich ice grains close to the jets ejected from the moon.

Data from Cassini's cosmic dust analyzer show the grains expelled from
fissures, known as tiger stripes, are relatively small and usually
low in salt far away from the moon. But closer to the moon's surface,
Cassini found that relatively large grains rich with sodium and
potassium dominate the plumes. The salt-rich particles have an
"ocean-like" composition and indicate that most, if not all, of the
expelled ice and water vapor comes from the evaporation of liquid
salt-water. The findings appear in this week's issue of the journal Nature.

"There currently is no plausible way to produce a steady outflow of
salt-rich grains from solid ice across all the tiger stripes other
than salt water under Enceladus's icy surface," said Frank Postberg,
a Cassini team scientist at the University of Heidelberg, Germany,
and the lead author on the paper. When water freezes, the salt is
squeezed out, leaving pure water ice behind. If the plumes emanated
from ice, they should have very little salt in them.

The Cassini mission discovered Enceladus' water-vapor and ice jets in
2005. In 2009, scientists working with the cosmic dust analyzer
examined some sodium salts found in ice grains of Saturn's E ring,
the outermost ring that gets its material primarily from Enceladean
jets. But the link to subsurface salt water was not definitive.

The new paper analyzes three Enceladus flybys in 2008 and 2009 with
the same instrument, focusing on the composition of freshly ejected
plume grains. The icy particles hit the detector target at speeds
between 15,000 and 39,000 mph (23,000 and 63,000 kilometers per
hour), vaporizing instantly. Electrical fields inside the cosmic dust
analyzer separated the various constituents of the impact cloud.

The data suggest a layer of water between the moon's rocky core and
its icy mantle, possibly as deep as about 50 miles (80 kilometers)
beneath the surface. As this water washes against the rocks, it
dissolves salt compounds and rises through fractures in the overlying
ice to form reserves nearer the surface. If the outermost layer
cracks open, the decrease in pressure from these reserves to space
causes a plume to shoot out. Roughly 400 pounds (200 kilograms) of
water vapor is lost every second in the plumes, with smaller amounts
being lost as ice grains. The team calculates the water reserves must
have large evaporating surfaces, or they would freeze easily and stop
the plumes.

"This finding is a crucial new piece of evidence showing that
environmental conditions favorable to the emergence of life can be
sustained on icy bodies orbiting gas giant planets," said Nicolas
Altobelli, the European Space Agency's project scientist for Cassini.

Cassini's ultraviolet imaging spectrograph also recently obtained
complementary results that support the presence of a subsurface
ocean. A team of Cassini researchers led by Candice Hansen of the
Planetary Science Institute in Tucson, Ariz., measured gas shooting
out of distinct jets originating in the moon's south polar region at
five to eight times the speed of sound, several times faster than
previously measured. These observations of distinct jets, from a 2010
flyby, are consistent with results showing a difference in
composition of ice grains close to the moon's surface and those that
made it out to the E ring. The paper was published in the June 9
issue of Geophysical Research Letters.

"Without an orbiter like Cassini to fly close to Saturn and its moons
-- to taste salt and feel the bombardment of ice grains -- scientists
would never have known how interesting these outer solar system
worlds are," said Linda Spilker, NASA's Cassini project scientist at
the Jet Propulsion Laboratory (JPL) in Pasadena, Calif.

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The mission is
managed by JPL for NASA's Science Mission Directorate in Washington.

For more information about Cassini, visit:

http://www.nasa.gov/cassini

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